Semiconductor devices, such as a transistor, wiring and contact, used to compose an integrated circuit have been needed to be finer with an increase in integration density of integrated circuits. Also, a device isolation width for isolating between devices has been needed to be narrower.
LOCOS process has been generally used to isolate between semiconductor devices. In LOCOS process, bird's beak encroachment may occur during field oxidation. Due to the bird's beak encroachment, the dimensions of device isolation width and device width become difficult to control. Under the circumstances, pad oxide film formed under nitride film is thinned to reduce the bird's beak encroachment of the pad oxide film so as to obtain a fine device.
However, finer device isolation becomes difficult to realize since leakage current is increased by a stress applied to a substrate during oxidation due to bird's beak size and thinned pad oxide film.
Poly-buffer LOCOS process, where polysilicon film is inserted between nitride film and pad oxide film, has been proposed to solve this problem. By inserting the polysilicon film, the bird's beak encroachment can be reduced to relieve the stress applied to the substrate.
N. Shimizu et al., "A Poly-Buffer Recessed LOCOS for 256 Mbit DRAM Cells", IEDM92, pp.279-282, 1992 suggests a poly-buffer recessed LOCOS process, which is a modified poly-buffer LOCOS process, as a device isolation method applicable to 256 Mbit or more DRAM with a device isolation width of around 0.25 .mu.m.
The polysilicon film used to relieve the stress needs to be removed with the nitride film after the field oxidation so as to form a transistor, diffusion layer etc. Also, the polysilicon film and the pad oxide film formed thereunder need to be further thinned to realize a finer device isolation width. In general, it is known that there occurs a void in the buffer polysilicon film itself while it serves to relieve the stress to the substrate. In particular, the void occurs around the edge of pattern where the bird's beak extends.
Around this void, the pad oxide film located under the polysilicon film may be also etched when the nitride film is removed by etching. In this state, when the polysilicon film is etched, the substrate may be holed since the void occurring portion does not have the pad oxide film or the pad oxide film is thinned. This is because the substrate is of monocrystalline silicon that has little etch selectivity to the polysilicon film.
When the substrate is holed, the leakage current and resistivity are increased, and therefore the characteristics are reduced or dispersed not to meet the device characteristics required. Further, even if it meets the device characteristics, the aesthetic appearance of the active region is thereby deteriorated.
Also, a method of doping nitrogen into polysilicon film so as to prevent the void from occurring in the buffer polysilicon film is proposed in IEDM94. However, when the void is decreased, the function to relieve the stress is reduced. Further, an additional apparatus to dope nitrogen into the polysilicon film is required.